The present invention relates to a metal oxide film formation method and apparatus for forming a metal oxide film made of (Ba,Sr)TiO3 (barium strontium titanate; to be referred to as BST hereinafter) or Pb(Zr,Ti)O3 (to be referred to as PZT hereinafter) having high permittivity used for a semiconductor memory device.
Along with rapid development of the semiconductor integration technique, various components forming a semiconductor integrated device are being downsized, increased in speed, and integrated at high degrees. For example, in the field of semiconductor memory devices, demands arise for larger capacity in addition to the above requirements.
For example, in a DRAM (Dynamic Random Access Memory) as a representative of semiconductor memory devices, a capacitor as one of main building components must be downsized and increased in capacitance per unit area.
A dielectric often used as the capacitor film of a DRAM capacitor is generally a silicon oxide in terms of the semiconductor process. The silicon oxide has a permittivity of 10 or less and a capacitance of 4 fF/xcexcm2 per unit area. The silicon oxide used for a capacitor formation film cannot obtain large capacitance per unit area.
Recently, BST and PZT having higher permittivity than that of the silicon oxide receive a great deal of attention as the materials of the capacitor formation film so as to increase the capacitance of the capacitor per unit area.
The DRAM structure will be described briefly.
FIG. 7 shows part of a DRAM memory cell formed on an Si substrate 701. This DRAM memory cell is constituted by a transistor 702 and capacitor 710. The transistor 702 forming the memory cell is connected to the capacitor 710 via a plug 703 connected to a drain terminal. The plug 703 is formed in a contact hole formed in an interlevel insulating film 704 made of an insulator such as silicon oxide, and is made of polysilicon to which an impurity is doped to make the plug 703 conductive.
The plug 703 is connected to a storage node 705 serving as one of the electrodes of the capacitor 710. The storage node 705 connected to the plug 703 is formed on the flat interlevel insulating film 704, and is formed from a film of platinum, ruthenium oxide, or the like. The storage node 705 is connected to the plug 703 via a barrier film 703a of TiN or the like.
A cell plate 707 is formed on the interlevel insulating film 704 including the storage node 705 via a capacitor film 706. The storage node 705, capacitor film 706, and cell plate 707 constitute the capacitor 710. An upper interlevel insulating film 708 is formed on the capacitor 710. Although not shown, a word line and bit line respectively connected to the gate and source terminal of the transistor 702 are formed on the upper interlevel insulating film 708.
As described above, the dielectric capacitor film is formed on the DRAM memory cell so as to cover the storage node serving as one of the electrodes constituting the capacitor.
The first performance demanded for the capacitor film of the capacitor is high permittivity. Examples of the material having high permittivity are compounds containing Ba, Sr, Ti, Pb, Zn, Bi, and Ta as constituent elements. The second performance demanded for the capacitor film of the capacitor is small leakage current.
To manufacture a DRAM of 1 Gbits or more with high integration degree, not only a capacitor film is formed from a material of high permittivity, but also the capacitor is three-dimensionally formed to increase the capacitor area. To three-dimensionally form the capacitor, the storage node must be three-dimensionally formed to form a capacitor film uniform in film thickness, composition, and characteristics on not only the flat portion but also side wall of the storage node having the three-dimensional structure. Forming a uniform film (capacitor film) on both the flat portion and side wall of the three-dimensional structure requires a thin film formation method excellent in coverage on a complicated step shape.
An example of the thin film formation method excellent in step coverage is chemical vapor deposition (CVD). According to CVD, a compound material containing an element for forming a thin film must be carried as gas to a substrate on which the thin film is to be formed. The most preferable state of the CVD compound material is gas at room temperature. With the use of a CVD compound material which is gas at room temperature, the supply amount of compound material to a substrate on which a thin film is to be formed is determined by only the flow rate of the compound material gas, and thus supply of the compound material can be controlled with high precision. However, Ba, Sr, Ti, Pb, Zn, Bi, and Ta compounds forming high dielectrics or ferroelectrics cannot exist as gas at room temperature. These compounds are liquids or solids at room temperature.
Hence, in forming a material of high permittivity by CVD, a raw material is supplied by bubbling. Strictly speaking, a solid raw material is supplied by sublimation.
If a raw material is supplied by bubbling, the supply amount can be more stably controlled and easily increased. For this reason, a liquid raw material is more desirable than a solid raw material. Supply of a raw material by bubbling requires high vapor pressure such as a sufficiently high vapor pressure at room temperature or less if possible, and a large temperature difference between the evaporation temperature and the thermal decomposition temperature.
However, Ba, Sr, Ti, Pb, Zn, Bi, and Ta compounds hardly exhibit sufficiently high vapor pressure, and only generate vapor upon heating to some extent. Most of these compounds are organic metal compounds.
In terms of the presence of a liquid organic metal compound usable for CVD, BST as a solid solution of barium titanate (BaTiO3: BT) and strontium titanate (SrTiO3: ST) receives a great deal of attention as the above-mentioned DRAM capacitor film which can be formed by CVD.
The BST is a high-dielectric having a permittivity of 200 or more, and satisfies the first performance demanded for the DRAM capacitor film.
Barium, strontium, and titanium forming BST produce organic compounds, and a BST thin film can be formed by MOCVD (Metal Organic CVD). The thin film formation method using CVD is thermal CVD of forming a metal or compound film at a relatively low temperature (400 to 500xc2x0 C.) using as a raw material an organic metal compound (MO) which is thermally unstable and readily decomposes.
The BST thin film can be formed by CVD capable of attaining excellent step coverage, and satisfies the second performance demanded for the DRAM capacitor film.
This BST film is generally formed by MOCVD by heating a substrate subjected to thin film formation, and supplying an oxidation gas such as oxygen (O2) together with Ba(thd)2, Sr(thd)2, and Ti(O-iPr)2(thd)2 vapors to the heated substrate. Note that Ba(thd)2, Sr(thd)2, and Ti(O-iPr)2(thd)2 are barium source, strontium source, and titanium source.
Formation of a BST film by MOCVD includes two methods, i.e., a premix method of mixing source gases and an oxidation gas in advance and supplying the gas mixture to a heated substrate, and a postmix method of individually supplying source gases and an oxidation gas to a heated substrate. Of the two supply methods, the premix method is more desirable because the source gases and oxidation gas can be supplied at a predetermined mixing ratio, a film can be formed even on a large-area substrate with a uniform thickness and composition, and the film composition can be easily controlled.
Since the premix method mixes an oxidation gas in advance, each source gas and the oxidation gas readily react with each other immediately before a source gas supply port to readily produce various intermediates. Since intermediates are readily produced, unwanted products are generated before the gas reaches the substrate. The unwanted products enter a film growing on the substrate to degrade the film quality.
To reduce oxygen defects in a BST film, suppress mixture of by-products in the film, and improve the film quality, the partial pressure of the oxidation gas on the substrate surface subjected to film formation is preferably set high. To increase the partial pressure of the oxidation gas on the substrate surface, the diameter of a gas supply port for supplying a source gas to the substrate is set small, or the oxygen flow rate is increased to increase the flow speed of gas supplied via the gas supply port so as to allow a larger amount of oxidation gas to reach the substrate surface.
When a larger amount of oxidation gas is supplied by decreasing the diameter of the gas supply port, the internal pressure increases immediately before the gas supply port. A gasifying unit for gasifying an organic metal compound is difficult to gasify the organic metal compound, and readily clogs.
Since unwanted products generated by a mixture of an oxidation gas and source gas readily coagulate at the end portion of the small-diameter gas supply port, the end portion of the gas supply port readily clogs with the unwanted products. Moreover, the film quality of a formed metal oxide film is poor owing to the above problem.
To the contrary, the postmix method does not mix an oxidation gas and source gas, and is free from the above-mentioned problem caused by unwanted products. However, the postmix method is difficult to form a film having a uniform film composition on a large-area substrate. This problem for a large-area substrate becomes serious particularly when a large amount of oxygen must be supplied for film formation. In addition, the postmix method of individually supplying an oxidation gas and source gas to a substrate is difficult to control the film composition. For example, when a BST film is to be formed, oxygen as an oxidation gas must be supplied at a flow rate of about 1 SLM. However, if such a large amount of oxidation gas is supplied, a BST film formed on an 8xe2x80x3 substrate is nonuniform in film thickness and film composition.
In formation of a BST film by MOCVD, Ti is hardly contained in a film being formed, compared to Ba and Sr. The content of Ti in the formed film cannot reach a predetermined value, and a high-quality BST film is difficult to form. The phenomenon that Ti is hardly contained in a film being formed poses a serious problem particularly when a film is formed at a low temperature in order to reduce thermal stress on the underlayer.
It is, therefore, a principal object of the present invention to provide a formation method and apparatus for a metal oxide film excellent in uniformity and electrical characteristics.
According to an aspect of the present invention, there is provided a metal oxide film formation method comprising the steps of individually preparing a source gas mixture essentially consisting of organic compound gases containing at least three metals, and an oxidation gas, supplying the oxidation gas to a substrate set in a closed vessel at a predetermined pressure and then supplying the gas mixture while the substrate is heated, and forming a metal oxide film on the substrate.
According to another aspect of the present invention, there is provided a metal oxide film formation method comprising the steps of individually preparing a first source gas mixture essentially consisting of organic compound gases containing at least two metals, and a second source gas mixture obtained by mixing in advance an organic compound gas containing titanium and an oxidation gas, supplying the oxidation gas to a substrate set in a closed vessel at a predetermined pressure and then supplying the gas mixtures while the substrate is heated, and forming a metal oxide film on the substrate.
According to still another aspect of the present invention, there is provided a metal oxide film formation apparatus comprising a film formation chamber constituting a closed vessel, evacuation means for evacuating an interior of the film formation chamber to a predetermined vacuum degree, a substrate susceptor arranged in the film formation chamber to place a substrate on a surface of which a metal oxide film is to be formed, heating means for heating the substrate set on the substrate susceptor, first supply means for supplying a source gas mixture essentially consisting of organic compound gases containing at least three metals to the surface of the substrate set on the substrate susceptor, and second supply means for supplying an oxidation gas to the surface of the substrate set on the substrate susceptor.